The present invention relates to large scale integrated circuit manufacture and, more particularly, to an improved method and apparatus for bonding circuitry to a circuit pad supported by an improved base laminate that increases the resistance of the circuitry to deform.
In recent times, as large scale integrated circuits have become denser, the copper circuitry in the pads have become ever thinner. Working with such thin circuitry, placing and attaching them to appropriate sites, has thus become ever more challenging.
During assembly, the pads are supported by a mesh laminate, while a capillary tool is used to attach the fine wiring onto the pads. As the tip of the capillary tool wipes across the surface of the pad in order to clean it, then applies energy in the form of heat, pressure and vibration, or any combination thereof, while pushing the circuitry to bond or weld the wires to the pad, it has been discovered that pads collapse or deform. The collapse of the pads is due to the interaction of the gold wire thickness, copper hardness, total pad thickness, force of the capillary tool, and material construction of the base laminate. Previous solutions to this problem involved changing the wire bonding parameters, but this had the disadvantage of producing a weaker bonded product or, in fact, no bond at all.
Copper pads have reached a thickness of less than 0.7 mils in order to provide for finer circuit lines and greater density. Experiments revealed that with the gold and nickel parameters of the pad being held constant, changing the base laminate could resolve the problem.
After pad collapse, windows of relatively thick resin had been observed in the open weave of the laminate, where the glass strands of the laminate supported the pad. This led to the conclusion that changing the laminate from an open weave to a closed weave could decrease or eliminate the likelihood of pad collapse.
It was discovered that the closed weave laminate improved the interaction of the pad thickness, such that copper thicknesses as low as 0.4 mils could be used without pad failure. The inventive process reflects the discovery that mesh with a separation dimension between warp or weave strands, measured lengthwise through the laminate, must be less than or equal to the diameter of the gold wires, in order to avoid pad collapse. The closed weave of the laminate was observed to improve the interaction of the pad thickness for thinner applications.
In U.S. Pat. No. 4,848,639, issued on Jul. 18, 1989 to Belanger, Jr. for COMPLIANT PAD FOR USE IN TAPE AUTOMATED BONDING PROCESS, a work piece used in bonding the inner leads of an integrated circuit is illustrated. The work piece is constructed of a metallic base layer to which a compliant pad is affixed. A gas channel is provided for a source of heated nitrogen. A ceramic layer is positioned over the gas channel to provide for heat distribution and support of the integrated circuit during its bonding to an associated tape.
In U.S. Pat. No. 5,092,510, issued on Mar. 3, 1992 to Anstrom et al. for METHOD AND APPARATUS FOR CIRCUIT BOARD SUPPORT DURING COMPONENT MOUNTING, a temporary support fixture is depicted for mounting components to selected locations on a circuit board. The fixture features a flexible support to provide for variations and tolerance differences between the circuit board and placement head.
In U.S. Pat. No. 5,562,948, issued to Trepte et al. on Oct. 8, 1996 for METHOD AND APPARATUS FOR PRODUCING AN ELECTRICALLY CONDUCTIVE WALL FROM A FABRIC AND A SHEET MATERIAL, a fabric and film are provided with an insulating coating in the production of flexible containers.
In Japanese Publication No. JP 1222950, a laminated board is shown with a conventional glass cloth, epoxy resin and inorganic filler, used in printed circuit boards.
In Japanese Publication No. JP 62294546, a laminate is formed by impregnating a fluoro-resin fabric with a resin varnish comprising a polyimide or epoxy resin.
In German Publication No. DE 3716531, a laminated material is formed in a continuous process by passing a resin impregnated fabric and copper foil between a sandwiching set of rollers.
In Publication No. SU 1062233, a resin impregnated, heat resistant fabric laminate is fabricated from an arylene-phenol-formaldehyde resin, an epoxy-triphenol resin, a tris-(dimethyl aminomethyl)-phenol, and an organic solvent.
In accordance with the present invention, there is provided a process and apparatus for bonding circuitry to a circuit pad in large scale integrated circuitry. The circuit pad is supported by a closed weaved laminate base that increases the resistance of the pad to deform during the wire bonding process. The laminate comprises a woven, fiberglass mesh having a separation dimension between warp or weave strands, measured lengthwise through the laminate, less than or equal to that of the diameter of the gold wires that are being bonded to the copper pad. The diameter of the gold bonding wire is generally less than approximately 1.0 mil. The improved laminate base can accommodate circuit devices having copper pads whose thicknesses are as low as 0.4 mils, without pad deformation.
It is an object of this invention to provide an improved method and apparatus for fabricating circuit devices.
It is another object of the invention to provide an improved supporting base laminate in a wire bonding process in the manufacture of large scale integrated circuits.